Typical aircraft radar altimeters include separate reception and transmission antennas located on the bottom of the fuselage of commercial or private aircraft. Separate transmit and receive antennas have historically been used in order to provide isolation between the transmitter and receiver during continuous transmission and reception of a radar signal. Transmitter to receiver isolation was required due to technology shortcomings of microwave signal sources and microwave device packaging technology. Similarly, microwave sources used in present radar altimeters used open loop methods.
Operation of existing radar altimeters relies on a reflection of the transmitting antenna signal from the ground to the receiving antenna. At high altitudes, the separation distance between transmit and receive antennas results in a small reflection angle between the transmitted and received signals and provides excellent signal reception. At much lower altitudes, such as during a landing; the reflection angle between the transmitting and receiving antennas becomes very large, thus attenuating signal reception at the outer reaches of the antenna beamwidths. When the aircraft is below a low altitude threshold, the reflection angle will exceed the beamwidth of the transmitting or receiving antennas and altimeter operation will cease. Therefore, at low altitudes the separation distance between the two antennas of conventional radar altimeters reduces the received signal strength compromising signal-to-noise ratio and thus reduces altitude accuracy. Moreover, conventional dual antenna altimeters may erroneously acquire reflections from aircraft components such as engines and wheel gear instead of the correct ground reflection. When at low altitudes, a single antenna radar altimeter uses a single vertical reflection path to and from the ground not impacted by altitude or attitude of the aircraft. In special applications such as an aircraft tail-strike protection system, there is a requirement to measure distances to the ground of less than one foot. Hence, a dual antenna altimeter will not function in such applications. Therefore, there are many needs for a single antenna FM radar altimeter.
The U.S. Pat. No. 6,426,717 to Maloratski presents a single antenna FM radar altimeter that performs continuous wave (FM/CW) modulation as well as an interrupted continuous wave modulation. FIG. 1 illustrates Maloratski's radar altimeter and FIG. 2 illustrates phase noise produced by a comparable system. Maloratski includes a circulator that directs transmission signals to the antenna or directs received signals through a radar-processing portion. Maloratski connects the circulator to the antenna via a coax cable, as it is the intent of the patent to remotely locate the radio frequency generation components of the altimeter from the antenna. Precision low range altimeter applications require exceptionally stable altitude data. However, temperature and moisture may affect coax cables by increasing cable insertion loss, increasing reflection coefficients and changes in propagation delay times. Therefore, no means presently exist to continuously calibrate the true electrical length of the connecting cable. Any radar altimeter connected to its antenna or antennas via coax must calibrate propagation delay in order to determine a fixed distance to and from the transmitting and receiving antenna(s) caused by the electrical length of the coax for each aircraft installation.
Maloratski also presents closed-loop analog circuitry for continuously adjusting modulation rate to produce a constant frequency received signal but the loop does not control the linearity or phase noise of the radar modulation. Any frequency modulated radar altimeter relies upon a nearly ideal linear modulation function of frequency change versus time. Maloratski's closed-loop analog circuitry provides no means to verify that the modulation function is nearly ideally linear as a function of time, temperature or other environmental effects because it controls the frequency of the received signal only. In this way, Maloratski's approach uses an open loop modulation system.
Radio frequency sources of many types are subject to Frequency Pulling as a function of load impedance. As a result, open loop modulation systems suffer distortion in the linearity of the frequency modulation function due to a varying Voltage Standing Wave Ratio (VSWR) caused by coax cable deterioration and/or poor antenna matching. Poor modulation linearity results in degraded signal to noise ratio, altitude accuracy and causes errors in measurements of modulation rate.
Many conventional radar altimeters, including the single antenna altimeter proposed by Maloratski, continuously adjust the period of the linear frequency modulation waveform as a function of altitude in order to achieve a constant received difference frequency. This constant received difference frequency is key to the altitude tracking mechanism of Maloratski and most known radar altimeters. While this design feature provides a means to facilitate analog altitude tracking subsystems, it forces the altimeter to additionally provide an automatic gain control (AGC) circuit that adjusts the amplitude of the received signal as a function of altitude and reflection brightness from the ground. This design feature complicates the altimeter design and imposes limitations to the response time of the overall altimeter circuitry with rapidly varying ground heights.
A basic concern for Frequency Modulated/Continuous Wave (FM/CW) radars with a single antenna is a large signal reflection from its antenna or connecting coax. Large amplitude reflections from the antenna or connecting coax cause the continuously transmitting radar to jam itself, thereby limiting sensitivity. Maloratski and others have utilized specialized cancellation circuitry in an attempt to prevent FM/CW self-jamming.
Therefore, present single antenna radar altimeter systems, like Maloratski, are overly complex, utilize open loop modulation and are relatively imprecise because of time and temperature changes and degraded RF performance due to coax cable degradation over time.
Typically, all radar altimeters are located mid-ship on an aircraft in relatively close proximity to the antenna installations, in order to minimize coax losses at 4.3 GHz. Current radar altimeters have not been physically combined with any other navigation radio components. Consequently, the remotely located radar altimeter incurs a weight penalty because an extra box is needed and because many feet of heavy coax cable is used.
Therefore, there exists a need to reduce the weight of electronics on an aircraft while improving data analysis.
Multi-Mode Radio (MMR) systems have combined various navigation components, such as ILS, INS, GPS (GNSS), and other radios. Their use is limited to the data from one system backing up or verifying the data from another system. For example, when using INS data for an approach to landing, differentially corrected GPS data constantly corrects for INS data drift. If there is a loss of GPS data or GPS integrity falls below an acceptable value (due to satellite acquisition problems), INS data may only be used for short period.
Therefore, there exists a need to provide greater integrity of INS and GPS data for use in various navigation scenarios.